This invention relates to steganography, to copy protection and to applying and detecting digital watermarks in video images.
The advent of technology for storing images on digital media has increased the need for a method to protect against piracy. Images stored on prior forms of media (e.g. VHS, Beta, audiotapes, etc.) are inherently degraded when copied. Images stored on digitally encoded media can be copied with no degradation; therefore, perfect copies of copies of copies, etc. can be made.
The introduction of Digital Versatile Discs (DVD) containing movies has created increased incentives for both casual and professional unauthorized copying. At the movie industry""s urging, technology has been put in place to protect against simple duplication of DVD disks using equipment available to unsophisticated consumers. This is similar to the protection that exists which prevents one from duplicating a VCR tape by connecting together two commercially available VCRs.
While such protection mechanisms protect against some types of copying, a personal computer connected to a DVD device present a much more complicated problem. Open architecture devices such as personal computers reproduce the signals in the xe2x80x9cclearxe2x80x9d and such devices have many entry points, which can be used to duplicate material once it is in the xe2x80x9cclearxe2x80x9d. The present invention uses digital watermarks to address the above described problem. The present invention also has other applications.
It is known that to facilitate the detection of digital watermarks one can insert a watermark signal that forms a grid. The grid can be used to determine orientation and scale. With the present invention the data signal and the grid signal are integrated into a single watermark signal in such a manner that the visual artifacts introduced by the watermark are minimized.
In applications such as DVD, an important factor that needs be considered is the bit rate of the bit stream. There are disadvantages if introduction of a watermark into a bit stream changes the bit rate. For example if images are going to be recorded on a medium such as a DVD disc, increasing the number of bits in the bit stream will decrease the number of images that can be recorded on a single disk. It is known that, in general, adding a watermark to a stream of images will increase the number of bits in the bit stream. The present invention provides a method and apparatus, which preserves the bit rate even though watermarks are introduced into the images.
The well-known JPEG and MPEG data compression techniques transform images utilizing a discrete cosine transform (DCT) which produces a matrix of DCT coefficients. These coefficients are arranged into blocks (e.g. into 8 by 8 blocks of coefficients). The blocks of DCT coefficients are in turn arranged into macro blocks (e.g. into 16 by 16 arrays containing four 8 by 8 blocks). With the present invention selected DCT coefficients in each block are slightly increased or slightly decreased in response to a watermark signal. The changes in the blocks that comprise each macro block are done in a coordinated manner so that the phase of the watermark signal is preserved across the block boundaries. By preserving the phase across block boundaries, a detectable grid is formed which can be used as an orientation and scaling grid.
The present invention also maintains the bit rate of the image signal. The bit rate of the signal is preserved by maintaining a count (referred to as the cumulative change count) that represents the amount that the bit rate has been increased by changes in coefficients less the amount that the bit rate has been decreased by changes in the coefficients. If at any time the cumulative change count exceeds a pre-established limit, coefficient changes that decrease the cumulative change count continue; however, coefficient changes that increase the cumulative change count are suspended. The suspension of coefficient changes that increase the cumulative change count continues until the cumulative change count falls below the pre-established limit. The above described process can be described as selectively changing the intensity of a watermark signal in a bit stream so as to prevent the entropy of the combined signal from exceeding a pre-established limit.